Loss of separation between 2 B747's when the higher one, a B747-200 with pms engaged, descends to within 600 ft vertically from the lower one in zan airspace.

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Attributes
ACN	616328
Time
Date	200405
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	intersection : nippi
State Reference	FO
Altitude	msl bound lower : 34600 msl bound upper : 35400
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zan.artcc
Operator	common carrier : air carrier
Make Model Name	B747-200
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Route In Use	enroute : pacific enroute airway : r220.airway
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zan.artcc
Operator	common carrier : air carrier
Make Model Name	B747-400
Operating Under FAR Part	Part 121
Flight Phase	cruise : enroute altitude change
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 16000 flight time type : 8500
ASRS Report	616328
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude conflict : airborne critical non adherence : required legal separation non adherence : clearance other anomaly
Independent Detector	other flight crewa
Resolutory Action	aircraft : automation overrode flight crew flight crew : took evasive action flight crew : overrode automation flight crew : overcame equipment problem flight crew : became reoriented flight crew : returned to assigned altitude
Miss Distance	horizontal : 0 vertical : 600
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation
Situations
Airspace Structure	class a : zan.a

Narrative:

At cruise FL350, pms engaged at mach .85. B747-400 directly below at FL340 in cruise. Sequence of events: 1) radio altimeter came alive, followed in 'rapid' succession by: a) GPWS 'pull up, terrain, terrain,' B) autoplt disconnect, C) attempt to re-engage autoplt fails (basic mode). 2) aircraft commenced descent, followed by climb RA. 3) immediately pulled back on control column. Bottom of descent approximately FL346.5. Followed climb RA to FL354, horizontal separation gained on traffic. Note: zan 'watch center' advised of this incident. Apparently other airlines flying 'classic' -200 B747's have had similar autoplt disconnects, with an aircraft directly below them. In pms modes, an automatic disconnect may occur (as per my incident). Corrective action may be, on rvsm airspace with 1000 ft vertical separation, to have a 1 NM horizontal separation for non FMS equipped aircraft.

Original NASA ASRS Text

Title: LOSS OF SEPARATION BTWN 2 B747'S WHEN THE HIGHER ONE, A B747-200 WITH PMS ENGAGED, DSNDS TO WITHIN 600 FT VERTLY FROM THE LOWER ONE IN ZAN AIRSPACE.

Narrative: AT CRUISE FL350, PMS ENGAGED AT MACH .85. B747-400 DIRECTLY BELOW AT FL340 IN CRUISE. SEQUENCE OF EVENTS: 1) RADIO ALTIMETER CAME ALIVE, FOLLOWED IN 'RAPID' SUCCESSION BY: A) GPWS 'PULL UP, TERRAIN, TERRAIN,' B) AUTOPLT DISCONNECT, C) ATTEMPT TO RE-ENGAGE AUTOPLT FAILS (BASIC MODE). 2) ACFT COMMENCED DSCNT, FOLLOWED BY CLB RA. 3) IMMEDIATELY PULLED BACK ON CTL COLUMN. BOTTOM OF DSCNT APPROX FL346.5. FOLLOWED CLB RA TO FL354, HORIZ SEPARATION GAINED ON TFC. NOTE: ZAN 'WATCH CTR' ADVISED OF THIS INCIDENT. APPARENTLY OTHER AIRLINES FLYING 'CLASSIC' -200 B747'S HAVE HAD SIMILAR AUTOPLT DISCONNECTS, WITH AN ACFT DIRECTLY BELOW THEM. IN PMS MODES, AN AUTOMATIC DISCONNECT MAY OCCUR (AS PER MY INCIDENT). CORRECTIVE ACTION MAY BE, ON RVSM AIRSPACE WITH 1000 FT VERT SEPARATION, TO HAVE A 1 NM HORIZ SEPARATION FOR NON FMS EQUIPPED ACFT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.